The present invention broadly relates to telecommunication systems, and especially those employing frequency sweep devices. More particularly, the invention deals with a wide bandwidth device for demodulating frequency division multiplexed (FDM) signals.
Telecommunications systems for digital data often use the technique of transmitting the data from different sources or users on signals of different frequency, more commonly known as FDMA (Frequency Division Multiple Access). In FDMA, each signal of "channel" of a user is assigned a discrete portion of the transmitted-frequency spectrum so that many channels can be transmitted over a single transmission medium. Typically, PSK (Phase Shift Keying) techniques are employed in which a constant amplitude carrier has its phase angle varied in accordance with the digital intelligence from the corresponding source.
In the past, the transmitted information has been recovered at a receiver using a separate PSK demodulator for each of the FDMA channels in order to provide individually accessible streams of data associated with each transmitting source thereof. These previous demodulators sometimes include so-called "fast Fourier transformers" (FFT's) for transforming the frequency division multiplexed signal into a time division multiplexed signal. FFT's of this type perform processing of the signal after it has been converted from analog to digital form. Typically, FFT's employ digital multipliers, ROM-stored tables and "butterfly" digital connections in order to perform the signal conversion. The bandwidth of these FFT's is limited by the conversion rate of the associated analog-to-digital convertor and the system clock rate. It is, primarily, the relatively limited bandwidth of FFT's which necessitates the use of separate demodulators for demodulating each channel of a frequency division multiplexed signal. The use of separate PSK demodulators to accommodate each of the FDMA channels, while adequate for some purposes, is impractical for other applications where a receiver's weight, power and complexity are critical factors, as for example in regenerative satellites.
The present invention overcomes the problems associated with the prior art receivers by converting N channels of FDMA data into individually accessible streams of demodulated data, using a single, highly accurate, wide bandwidth demodulator.